The present invention relates to a new and useful nulling loop configuration for a cantilever magnetometer which can be used to measure magnetic properties of a sample specimen, and in particular to a high sensitivity magnetometer having a cantilever sensing element which can be used in a wide range of magnetic fields and temperatures without any significant degradation in accuracy.
Methods and devices for measuring a magnetic moment of a single crystal sample in a magnetic field using cantilever capacitance have been in existence for about 20 years, while methods for measuring the magnetic properties of single crystals using magnetic torque have been known for about 60 years. For example, a discussion of one type of magnetic torque balance is provided by F. B. Humphrey, et al., in "Sensitive Automatic Torque Balance for Thin Magnetic Film," Rev. Sci. Instr., v.34, p.348 (1963). An early discussion of torque magnetometers can be found in Williams, "Some Uses of the Torque Magnetometer," Rev. Sci. Instr. v.8, p.56 (1936).
There are two cases which can occur when a sample crystal is subjected to a magnetic field. If the sample material has an anisotropic magnetic moment, a uniform magnetic field will cause a magnetic torque, .tau.=m.times.B, (where m is the magnetization or anisotropic susceptibility, and B is the magnetic flux of the field) to be exerted on the sample, thereby causing a torsional displacement of the sample.
If the sample material is isotropic in nature, then a uniform magnetic field will not generate a magnetic torque, and instead, a magnetic field gradient .gradient.B is required. The force generated is F=m.multidot..gradient.B, where m is the magnetization of the sample. The force then causes a displacement of the sample.
These magnetometry techniques are often employed with superconducting materials and used in high magnetic fields and/or low temperatures. Articles discussing such uses include, Brooks, et al., "Small sample magnetometers for simultaneous magnetic and resistive measurements at low temperatures and high magnetic fields," Rev. Sci. Instr. v.58, p.117 (1987), and, Qvarford, et al., "Microtorquemeter for magnetization measurements on small superconducting samples," Rev. Sci. Instr. v.63, p.5726 (1992).
It is also known to use pulsed magnetic fields, which are generated by the rapid release of large amounts of energy through a coil. The resulting magnetic field reaches a peak value within a very short time--usually 0.01 to 0.1 seconds--and immediately thereafter returns to zero. Pulsed fields are advantageous for use with cantilever magnetometry techniques because the very short periods in which the magnetic fields are generated do not induce as much thermal heating in the coil windings. Since a current is only applied in short, rapid bursts in a pulsed coil, much larger currents may be used without melting the coil. Thus, greater magnitude magnetic fields can be generated using pulsed magnetic fields without damage to conventional equipement. Currently, pulsed fields in the range of 70 Tesla can be generated, and it is expected that 100 Tesla fields will be possible in the near future. These field strengths are about 3 times greater than fields generated using conventional DC magnetic field generation techniques.
Cantilever magnetometry technology is used to measure the magnetic properties of the sample using the relationship between the magnetic force (or torque) and the displacement of a cantilever beam caused by the force or torque exerted on the sample.
The sample is placed on a cantilever and subjected to a magnetic field. Reaction between the magnetism of the sample and the magnetic field causes a displacement of the sample which can be measured in one of several ways. The displacement is proportional to the magnetic properties of the sample.
One known device which uses capacitance for measuring the displacement is disclosed in the Brooks article, supra. It measures the capacitance change between two electrodes caused by displacement of the sample.
A commercially available device which is commonly used to measure magnetic moments in low strength magnetic fields is the superconducting quantum interference device (SQUID). This device's sensitivity is approximately 10.sup.-9 Joules/Tesla in a range of magnetic field strengths from 0.01 Tesla to about 7 Tesla. At increasingly higher magnetic field strengths, this device's sensitivity can decrease considerably due to interference from high magnetic fields. The Qvarford article, supra, discusses other limitations of the superconducting quantum interference device.
Another, more advanced device for measuring magnetic properties of superconducting materials is disclosed in an article by Chaparala, et al., entitled "Capacitance Platform Magnetometer for Thin Film and Small Crystal Superconductor Studies" AIP Conf. Proc., p.407 (1993).
The device disclosed by Chaparala, et al., is a silicon wafer cantilever with gold lead wires on a top side, and a metallized bottom side, electrically connected to a capacitance bridge. The bottom side of the wafer acts as the upper plate of a capacitor. The bottom plate of the capacitor is a metallized plate separate from the wafer. The gold lead wires form a symmetrical pair of nulling loops on the top side. Several single electrical conductivity leads extend along the cantilever top side between the loops. A sample is placed in between the nulling loops in electrical contact with the single leads.
Both magnetic torque and magnetic force may be measured with the device. However, the sensitivity of this device, while better than other known devices, such as the superconducting quantum interference device (SQUID), is still limited by noise and leakage currents between the capacitance and the nulling loop circuits and to the specimen. Temperature differences can also affect the sensitivity of the Chaparala, et al. device.
Nulling loops such as the type described by Chaparala, et al. have unbalanced segments which generate a mixture of force and torque components due to interaction with the magnetic field, even when the sample and magnetic field are specifically arranged to use the cantilever to detect either torque or force. Typically, the ratio is about 80% of one component and 20% of the other. For example, when the sample and magnetic field are designed to generate only a torque component, the unbalanced nulling loops will generate both torque and force components in a ratio of about 80% torque to 20% force. Thus, while the torque cantilever may be nulled, the calibration is not 100% accurate due to the unbalanced segments interaction with the magnetic field.